To experimentally investigate reinnervation and synaptogenesis in excitotoxically-damaged cochleae, we developed an organotypic cochlear explant in which a portion of the organ of Corti and corresponding portion of the spiral ganglion are removed intact, maintaining normal morphology and synaptic interactions. Briefly treating the explant with high levels of glutamate agonists results in excitotoxic degeneration of inner hair cell (IHC) - type I spiral ganglion neuron (SGN) synapses but does not affect hair cell or SGN viability. The synapses regenerate but the restored innervation is aberrant: the number of synapses is reduced, and individual SGN axons contact multiple IHCs. In all these respects, the in vitro model mimics what has been observed following noise or glutamatergic excitotoxic damage in vivo. Exogenous neurotrophins - BDNF or NT-3 - significantly improve recovery: the number of synapses on IHCs is increased, synapse number is increased, and innervation of multiple IHCs by single axons is reduced.
In Aim 1, we quantitatively compare the ability of BDNF and NT-3 to promote regeneration with an extended recovery period and seek to improve our model by extending it to older animals. Our core set of experiments in Aims 2-4 use molecular genetic approaches, including the use of transgenic mice, to test specific hypotheses, suggested by our preliminary data, regarding the function of neurotrophins in recovery and reinnervation of IHCs after excitotoxic trauma.
In Aim 2 we test whether NT-3, the endogenous neurotrophin, acts in a highly spatially restricted manner to maintain synapses on individual IHCs. We will delete NT-3 from a small number of IHCs or inhibit TrkC function in a small number of SGNs and quantitatively compare these with their unmodified neighbors.
In Aim 3, we replace NT-3 with BDNF to test the hypothesis that NT-3 has a distinctive function in maintaining IHCSGN synapses and BDNF can't substitute. Finally, in Aim 4, we use p75NTR knockout mice to test the hypothesis that the neurotrophin receptor p75NTR promotes reinnervation after excitotoxic trauma. We will also assay post-trauma expression of p75NTR and putative ligands and test a specific mechanism: whether p75NTR promotes reinnervation by upregulating NT-3.

Public Health Relevance

In humans, noise experienced in youth accelerates hearing loss in aging. Noise is known to acutely damage spiral ganglion neuron (SGN) synapses in rodent models. This compromises SGN survival in the long-term, causing hearing loss long after the original exposure. Our studies aim to prevent delayed consequences of noise damage to SGNs by investigating the mechanisms that promote recovery of damage to SGN synapses.

Agency
National Institute of Health (NIH)
Institute
National Institute on Deafness and Other Communication Disorders (NIDCD)
Type
Research Project (R01)
Project #
5R01DC009405-03
Application #
8470153
Study Section
Auditory System Study Section (AUD)
Program Officer
Freeman, Nancy
Project Start
2011-06-07
Project End
2016-05-31
Budget Start
2013-06-01
Budget End
2014-05-31
Support Year
3
Fiscal Year
2013
Total Cost
$304,831
Indirect Cost
$102,956
Name
University of Iowa
Department
Biology
Type
Schools of Arts and Sciences
DUNS #
062761671
City
Iowa City
State
IA
Country
United States
Zip Code
52242
Bailey, Erin M; Green, Steven H (2014) Postnatal expression of neurotrophic factors accessible to spiral ganglion neurons in the auditory system of adult hearing and deafened rats. J Neurosci 34:13110-26
Kopelovich, Jonathan C; Cagaanan, Alain P; Miller, Charles A et al. (2013) Intracochlear electrical stimulation suppresses apoptotic signaling in rat spiral ganglion neurons after deafening in vivo. Otolaryngol Head Neck Surg 149:745-52
Green, Steven H; Bailey, Erin; Wang, Qiong et al. (2012) The Trk A, B, C's of neurotrophins in the cochlea. Anat Rec (Hoboken) 295:1877-95
Provenzano, Matthew J; Minner, Sarah A; Zander, Kaitlin et al. (2011) p75(NTR) expression and nuclear localization of p75(NTR) intracellular domain in spiral ganglion Schwann cells following deafness correlate with cell proliferation. Mol Cell Neurosci 47:306-15
Lu, Yuan; Zha, Xiang-ming; Kim, Eun Young et al. (2011) A kinase anchor protein 150 (AKAP150)-associated protein kinase A limits dendritic spine density. J Biol Chem 286:26496-506
Dagda, R K; Gusdon, A M; Pien, I et al. (2011) Mitochondrially localized PKA reverses mitochondrial pathology and dysfunction in a cellular model of Parkinson's disease. Cell Death Differ 18:1914-23
Atkinson, Patrick J; Cho, Chang-Hyun; Hansen, Marlan R et al. (2011) Activity of all JNK isoforms contributes to neurite growth in spiral ganglion neurons. Hear Res 278:77-85
Merrill, Ronald A; Dagda, Ruben K; Dickey, Audrey S et al. (2011) Mechanism of neuroprotective mitochondrial remodeling by PKA/AKAP1. PLoS Biol 9:e1000612
Wang, Qiong; Green, Steven H (2011) Functional role of neurotrophin-3 in synapse regeneration by spiral ganglion neurons on inner hair cells after excitotoxic trauma in vitro. J Neurosci 31:7938-49